1. Field of the Invention
The present invention relates to a solid-state imaging device employing a filter and relates to an electronic apparatus such as a digital-still camera and a video camera which employ the solid-state imaging device.
2. Description of the Related Art
In recent years, the miniaturization of the size of a pixel used in a solid-state imaging device has been making progress in order to allow the number of pixels used in the solid-state imaging device to be increased. Accompanying the miniaturization of the size of the pixel, an area occupied by the pixel also decreases so that the quantity of light incident on the pixel also becomes smaller as well. In addition, since an area occupied by a photodiode employed in the pixel also becomes narrow, the number of photons accumulated in the photodiode also becomes smaller as well and sensitivity deteriorates. Thus, the magnitude of a saturated signal generated in a saturated state of the pixel decreases as well. The sensitivity of a pixel employed in a solid-state image taking device is defined as the ratio of the magnitude of a signal generated by the pixel due to light incident on the pixel to the quantity of the incident light as will be explained later by referring to a diagram of FIG. 4. A high sensitivity of a pixel employed in a solid-state image taking device gives rise to a decrease in dynamic range as will be described later by referring to diagrams of FIGS. 4 and 9. That is to say, a high ratio of the magnitude of a signal generated by a pixel due light incident on the pixel to the quantity of the incident light results in a decrease in dynamic range.
In general, in comparison with a camera making use of a silver salt film, a digital-still or video camera employing a solid-state imaging device has a narrow dynamic range. The dynamic range is a brightness range in which an image can be taken without causing white and black failures. However, a monitoring camera is demanded to be capable of taking an image throughout a range from a dark place to a bright place. In addition, a monitoring camera is even demanded to be also capable of taking an image also at a dark place without causing a failure.
The dynamic range of a camera employing a solid-state imaging device is normally determined by a pixel which has the highest sensitivity. In a camera employing a solid-state imaging device having filters of elementary colors such as the G (green), R (red) and B (blue) colors for example, the dynamic range D of the G (green) pixel having the highest sensitivity normally determines the dynamic range of the camera. In the case of a solid-state imaging device having a saturated signal magnitude which is uniform for all pixels, a pixel having a highest sensitivity is the first pixel with its output signal attaining the magnitude of the saturated signal at an earliest time. This is because the pixel having the highest sensitivity is a pixel with a filter having the highest transmittance (that is, a filter transmitting a largest quantity of light) so that a largest number of protons are accumulated in a photo diode included in the pixel, causing a signal output by the pixel to attain the magnitude of the saturated signal at an earliest time.
There has been proposed an existing technology for improving the sensitivity of the solid-state imaging device. In accordance with the proposed existing technology, GRB (Green, Red and Blue) pixels of a Bayer array including a layout of filters of elementary colors (that is, the GRB colors) are used as pixels of a YRB array by making use of a Y pixel including a filter not absorbing light with visible wavelengths as a substitute for the G pixel. In addition, there has also been proposed another existing technology for improving the sensitivity of the solid-state imaging device. In accordance with the other proposed existing technology, one of G pixels placed in a checker board state in the Bayer array is replaced by the Y pixel described above. For more information on the existing technologies for improving the sensitivity of the solid-state imaging device, the reader is advised to refer to documents such as Japanese Patent Laid-open No. 2008-205940.
In the mean time, there has been proposed a existing technology for widening the dynamic range. In accordance with this proposed existing technology, two images are synthesized. The two images are a high-sensitivity image and a low-sensitivity image which have brightness levels different from each other. For more information on the existing technology for widening the dynamic range, the reader is advised to refer to documents such as Japanese Patent Laid-open No. 2004-56568.